1. Field of the Invention
The present invention generally relates to differential amplifiers. More specifically, the present invention provides a high gain differential preamplifier having an adjustable common-mode output level that can be adjusted to drive an output circuit.
2. Background Art
A differential preamplifier is often incorporated into the front-end of a differential receiver to increase overall receiver sensitivity. Specifically, a differential preamplifier is used to amplify or boost the signal strength of low power received signals so that the next stage differential receiver components operate as designed.
To adequately amplify a low power received signal, differential preamplifiers must be designed to achieve high gain. Further, differential preamplifiers are usually required to operate over a large bandwidth and to introduce minimal offset. To meet these stringent requirements, differential preamplifiers use transistors as active loads to boost overall gain and to minimize offset. Introduction of transistor loads, however, often results in low common-mode output voltage levels. If the common-mode output voltage level is too low, the differential preamplifier will not be able to power or drive the next stage of a differential receiver front-end that is coupled to the output of the differential preamplifier.
One method of increasing common-mode output levels without sacrificing the desirable features of active transistor loads is to increase the physical size of the transistors. Increasing the physical size of the transistors, however, increases the capacitance of the transistor loads. In turn, the bandwidth of the differential preamplifier is decreased. Alternatively, common-mode output levels can be boosted by increasing voltage supply levels of the differential preamplifier. As the desire for low power differential receiver applications has risen, however, the ability to supply differential preamplifiers with high voltage supplies has become impractical. There is therefore a need for a differential preamplifier design that uses small-sized transistor loads and low power voltage supplies, yet remains capable of providing a common-mode output level that can drive follow-on circuits.